Many processes, for example combustion processes or the industrial production of nitric acid or caprolactam, result in an offgas laden with nitrogen monoxide NO, nitrogen dioxide NO2 (referred to collectively as NOX), and dinitrogen monoxide N2O. While NO and NO2 have long been known as compounds of relevance for environmental toxicology (acid rain, smog formation) and global limits have been fixed for the maximum permissible emissions thereof, dinitrogen monoxide has also gained increasing attention in the field of environmental conservation in the last decade, since it contributes to a not inconsiderable degree to the degradation of stratospheric ozone and to the greenhouse effect. For reasons of environmental conservation, there is therefore an urgent need for technical solutions for eliminating dinitrogen monoxide emissions together with the NOX emissions.
There are already numerous known means of eliminating N2O on the one hand and NOX on the other hand.
In the case of NOX reduction, the selective catalytic reduction (SCR) of NOX by means of ammonia in the presence of vanadium-containing TiO2 catalysts should be emphasized (cf., for instance, G. Ertl, H. Knözinger, J. Weitkamp: Handbook of Heterogeneous Catalysis, vol. 4, pages 1633-1668, VCH Weinheim (1997)). According to the catalyst, this can proceed at temperatures of about 150 to about 450° C., and is conducted on the industrial scale preferably between 200 and 400° C., especially between 250 and 350° C. It is the most commonly used variant for reducing NOX levels in offgases from industrial processes and, given appropriate dimensions of the catalyst beds, enables an NOX decomposition of more than 90%.
There are also processes for reducing NOX that are based on zeolite catalysts, which proceed using a wide variety of different reducing agents. As well as Cu-exchanged zeolites (cf., for example, EP-A-914,866), iron-containing zeolites in particular appear to be of interest for practical applications.
For instance, U.S. Pat. No. 5,451,387 and EP-A-756,891 describe processes for selective catalytic reduction of NOX with NH3 over iron-exchanged zeolites, which work preferentially at temperatures between 200 and 550° C., especially around 400° C.
In contrast to reducing NOX levels in offgases, which has been established in industry for many years, there exist only comparatively few industrial processes for N2O elimination, which are usually aimed at a thermal or catalytic decomposition of the N2O. An overview of the catalysts, which have been demonstrated in principle to be suitable for decomposition and for reduction of dinitrogen monoxide, is given by Kapteijn et al. (Kapteijn F. et al., Appl. Cat. B: Environmental 9 (1996) 25-64). The catalytic decomposition of dinitrogen monoxide to N2 and O2 offers the advantage over catalytic reduction with selected reducing agents, such as NH3 or hydrocarbons, that no costs arise for the consumption of reducing agents. However, an effective reduction in N2O levels based on a catalytic breakdown, in contrast to reduction of N2O or else NOX, can be achieved effectively with the customary breakdown catalysts only at temperatures greater than 400° C., preferably greater than 450° C.
Again, transition metal-laden zeolite catalysts appear to be particularly suitable for catalytic breakdown of the N2O to N2 and O2 (U.S. Pat. No. 5,171,553).
Iron-laden zeolite catalysts are described as especially advantageous (for example in EP-A-955,080 or WO-A-99/34,901). The activity of the Fe-zeolite catalysts for N2O breakdown is enhanced considerably by the simultaneous presence of NOX, as demonstrated scientifically, for example, by Kögel et al. in Catalysis Communications 2 (2001) 273-276 or by Perez-Ramirez et al. in Journal of Catalysis 208 (2003) 211-223.
This property appears to apply exclusively to iron-doped zeolites. Zeolites doped with other transition metals such as copper or cobalt do not show this behavior.
In many cases, N2O breakdown is actually inhibited by the presence of NOX, as known, for example, from Applied Catalysis B: Environmental 9 (1996) 25-64 [ch. 5.1], Applied Catalysis B: Environmental 12 (1997) 277-286 and from Catalysis Today 35 (1997) 113-120. This relates, for example, to Cu-, Co- and Rh-containing catalysts, which, in the absence of NOX, exhibit a very high activity for N2O breakdown, but in the presence of NOX have a distinctly reduced activity. Catalysts of this kind are referred to hereinafter as “NOX-sensitive”.
As well as the aforementioned catalysts and methods for NOX reduction and for N2O breakdown, the patent literature also describes combined methods for elimination of NOX and N2O. These are, for example, methods based on a catalytic reduction of NOX with NH3 (in a deNOX stage) and a catalytic breakdown of N2O to N2 and O2 over iron-containing zeolite catalysts (in a deN2O stage).
For example, WO-A-01/51,182 describes a method for eliminating NOX and N2O from the residual gas from nitric acid production, wherein the offgas to be cleaned is passed first through a deNOX stage and then through a deN2O stage with iron-laden zeolite catalysts. In the upstream deNOX stage, the NOX content is reduced to such an extent that an optimal NOX/N2O ratio of 0.001 to 0.5 is established, which leads to accelerated N2O decomposition in the downstream deN2O stage. Details of the apparatus configuration of this method are not disclosed.
The sequence of process stages described in WO-A-01/51,182 is very advantageous from a process or chemical engineering point of view, since the method is arranged in a rising temperature profile in the residual gas from the nitric acid production, between the absorption tower and residual gas turbine; in other words, the residual gas at first, prior to entry into the deNOX stage, has a low inlet temperature of <400° C., preferably <350° C., such that it is also possible to use conventional deNOX catalysts based on V2O5—TiO2. After the deNOX stage, prior to entry into the deN2O stage, there is then a (single) heating operation of the residual gas up to 350 to 500° C., such that effective catalytic N2O breakdown is possible. The offgas is then sent to a residual gas turbine in which the heat content of the offgas is recovered with decompression and cooling of the offgas.
A reverse connection of the two method stages, i.e. in a sequence in which first the N2O decomposition is envisaged, and then the NOX decomposition is effected, is also possible, as taught in WO-A-97/10,042, WO-A-01/51,181, WO-A-03/105,998 and WO-A-2006/119,870. WO-A-01/51,181 gives a detailed description not just of the method but also of an apparatus for conduction thereof. The latter is characterized by a sequence of two series-connected catalyst beds, with radial flow of the gas through at least one of them, and with the obligatory presence, between the catalyst beds, of an apparatus for introduction of a gaseous reducing agent into the gas stream leaving the first catalyst bed. In this method, the offgas is passed typically at a homogeneous temperature of <500° C. through two reaction zones containing iron-laden zeolite catalysts, which may be spatially separate from one another or connected to one another. In this method, N2O breakdown is effected first in the deN2O stage at an unreduced NOX content, i.e. with full exploitation of the co-catalytic NOX effect on the N2O breakdown, and then, after intermediate addition of ammonia, catalytic NOX reduction is effected. Since the NOX reduction should preferably proceed at the same temperature as the N2O breakdown, Fe-zeolite catalysts are likewise used in the deNOX stage, these catalysts, in contrast to conventional SCR catalysts, for example V2O5—TiO2-based catalysts, also being operable at higher temperatures >400° C. Intermediate cooling of the process gas is therefore not required.
Finally, JP-A-06/126,177 discloses the combined elimination of NOX and N2O based on a catalytic reduction of the NOX with NH3 (in a deNOX stage) and a catalytic breakdown of N2O to N2 and O2 (in a deN2O stage). The sequence of stages according to this document may be as desired. For the breakdown of the N2O, a supported catalyst is proposed, containing 0.001 to 2% by weight of metallic platinum or rhodium or metallic rhodium and copper. As well as these metals, iridium, ruthenium, iron, cobalt and nickel are also proposed. Support materials mentioned are aluminum oxide, silicon dioxide and zirconium dioxide, and also zeolites. Details of the selection of the catalysts for the reduction of NOX are not disclosed here.
The parallel chemical reduction of NOX and N2O has also already been described. In this context, it is known that the NOX reduction proceeds considerably more quickly than the N2O reduction. In this reduction method, a nitrogen-containing reducing gas, for example ammonia, is typically used for the NOX reduction, while the same reducing gas, such as ammonia, but also hydrogen, a hydrocarbon or carbon monoxide, is typically used for the N2O reduction. Examples of such methods can be found in WO-A-03/84,646 and in U.S. Pat. No. 4,571,326. The method according to U.S. Pat. No. 4,571,326 can also be conducted in one catalyst bed or in a sequence of a plurality of catalyst beds. Because of the relatively rapid reduction of the NOX, two zones form when one catalyst bed is used, with reduction principally of NOX in the first zone and reduction principally of N2O in the downstream, directly adjoining zone. This variant is shown, for example, in FIG. 4 of U.S. Pat. No. 4,571,329. FIG. 5 of U.S. Pat. No. 4,571,329 shows a sequence of two catalyst beds; these directly adjoin one another and form a zone in which principally NOX is reduced, followed by a zone in which principally N2O is reduced. Catalysts used for the N2O reduction are selected iron- or hydrogen-doped zeolites.
US-A-2002/0127163 describes a method for selective catalytic reduction of N2O with ammonia. Catalysts used are zeolites, which have preferably been doped with metals. This reduction method can be combined with an NOX reduction. FIG. 10 of this document demonstrates that methods of this kind can be conducted in one catalyst bed, or in a sequence of a plurality of catalyst beds. Accordingly, it is possible to conduct either a simultaneous reduction of NOX and N2O or else a first reduction of N2O followed by a reduction of the NOX. For catalytic reduction of N2O, a minimum amount of 0.5 mol of ammonia per mole of N2O is required. According to the description, the sequence of the reduction stages is controlled by the selection of the catalysts. A catalytic breakdown of the N2O to nitrogen and oxygen is explicitly not the subject of the invention disclosed.
The patent literature discloses reactors for a wide variety of different gas phase reactions including a sequence of at least two catalyst beds.
U.S. Pat. No. 2,475,855 describes a reactor for catalytic endo- or exothermic reactions, with a plurality of radial catalyst beds in the interior thereof. These are arranged separately from one another and have an axial line in which reactants are supplied to the catalyst and flow through it radially. The reverse flow direction is also possible. The reactor is used, for example, in the catalytic cracking of hydrocarbons.
U.S. Pat. No. 4,372,920 describes a reactor for heterogeneously catalyzed gas phase reactions, likewise with a plurality of radial catalyst beds in the interior thereof. These are arranged separately from one another and likewise have an axial line. The reactants flow axially through parts of the individual catalyst beds and radially through other parts of these catalyst beds. The reactor can be used, for example, for synthesis of ammonia or of methanol.
EP-A-1,022,056 describes a reactor for the treatment of fluids, comprising two directly adjoining beds of adsorbents or catalysts in a vessel. The beds consist of granules of different particle size, the lowermost bed having the coarser particle size. Arranged in between is a perforated plate, the holes of which have diameters greater than the diameter of the particles in the upper bed and smaller than the diameter of the lower bed. The reactor can be used for filtration, cleaning, separation and catalytic conversion of fluids.
U.S. Pat. No. 3,733,181 describes a reactor for the catalytic reduction of nitrogen oxides and for the catalytic oxidation of hydrocarbons and of carbon monoxide from offgases. The reactor comprises a combination of two concentric beds of catalysts for the two reactions, through which the offgas is passed in succession. Between the two beds, air is supplied to the offgas to be treated.
EP-A-967,006 discloses an apparatus for performance of catalytic reactions of a fluid in the gas phase. This comprises, in a reactor, an arrangement of two catalyst beds directly adjoining one another, each in essentially cylindrical form, with radial flow through one and axial flow through the other. This apparatus can be used, for example, in the desulfurization of natural gas.
To date, in commercial methods for combined reduction of NOX and breakdown of N2O in gases in the low to moderate temperature range at about 200 to 600® C., principally iron-doped zeolites are used. As described above, catalysts of this kind are notable in particular firstly for a very high activity for NOX reduction by means of ammonia and secondly for a high activity for breakdown of N2O, which is distinctly enhanced in the presence of NOX.
Other catalysts for the breakdown of N2O which are deactivated by the simultaneous presence of NOX can be used only under special conditions in industrial practice, i.e. in gases containing both NOX and N2O. It would be desirable if the use spectrum of such catalysts could be broadened, such that these catalysts could likewise be used in the removal of nitrogen oxides from offgases.
On the basis of the information available to date about catalysts other than iron-doped zeolites which would be usable for catalytic breakdown of N2O, a combined process for removal of nitrogen oxides from gases would be envisaged for these other catalysts, in which very substantial NOX reduction with ammonia, for example, takes place in a first stage, and then the remaining N2O would be broken down or reduced in a downstream stage. Such a more or less complete removal of the NOX in the first stage could be effected by the addition of appropriately large amounts of ammonia. In this case, however, when conventional SCR catalysts are used, for example those based on V2O5—TiO2, there is the risk that, in the case of limited amounts of catalyst, not the entire amount of added ammonia will in fact react with NOX, thus resulting in unwanted slippage of ammonia. This is problematic in the case of combined NOX reduction and N2O breakdown because the ammonia then gets into the downstream deN2O stage and, when zeolites not doped with transition metals are used, is oxidized at least partly to NOX, i.e. to NO and NO2. This in turn leads to partial inhibition or deactivation of the deN2O catalyst.
Moreover, it is known that the conventional SCR catalysts are generally usable only at temperatures of up to 400° C. In order to avoid stepwise heating of the gas stream to be treated and to enable a simple apparatus configuration, both stages of the nitrogen oxide degradation, i.e. the deNOX stage and the deN2O stage, should be operated at approximately equal temperatures.